Doug, why do you say “but”? The low ranking in figure 2 is the answer to the question in the title of the post.

I think that the reason for the low number of computer science papers in the proceedings of the AMS is simply that it wouldn’t occur to most of us to send a paper there, and the same may be true of other under-represented areas. I don’t think that the author is suggesting that the editors of those “generalist” journals are biased, but rather that the journals present a biased (in the statistical, not judgmental, sense) picture of the mathematical landscape.